The invention resides in the field of drawn glass whether it be drawn into ribbon, sheet, rod, tubing or other shapes. The drawing of glass is commonly accomplished by two standard processes including the drawing of glass while in a flowable semimolten state as it cools from a molten condition, either from a shaped orifice or mandrel or from the surface of a pool, and the reheating of a shaped glass piece to a semimolten condition and stretching that portion in the heated zone to attenuate the same to the desired dimensions and shape. Both such processes may be intermittent, but generally are performed in a continuous manner.
In the past it has been a problem during the drawing of glass to find an acceptable combination of furnace temperature, drawing tension and drawing speed. For example, a high temperature may be undesirable since the glass may become too fluid in the forming zone to maintain its desired shape, or because exotic or highly refractory materials of construction may be required when using such high temperatures. However, by reducing the glass temperature during the drawing process, the viscosity increases which results in an increase in the required drawing tension. Due to the increase in the glass viscosity, the drawing tension is often increased to the point where the freshly formed glass is overstressed resulting in breakage. In order to correct the breakage, the drawing speed may be reduced which automatically reduces tension, however extremely low and uneconomical production rates may result, and further devitrification may occur in some glass compositions.
The present invention reduces the problem of glass breakage during drawing and in many instances virtually eliminates such problem by introducing steam or water vapor into the atmosphere surrounding the glass while it is being formed.